Gear pumps are pumps with lower cost and less friction as compared with vane pumps and widely used as oil pumps and the like, for example, by being installed in automotive vehicles.
A gear pump generally uses a spur gear to avoid the generation of a thrust force.
Since gear pumps using a spur gear cause high pump noise due to an insufficient contact ratio of gears, it is not for an application required to be quiet (e.g. HEV, EV, etc.). Thus, it has been obliged to use more expensive vane pumps and the like, which has led to a cost increase.
On the other hand, gear pumps using a helical gear have an advantage of improving quietness while maintaining the same pump performance as compared with those using a spur gear.
However, a helical gear generates a thrust force in an axial direction and increases a frictional force between the gear and a pump body, wherefore there is a possibility of problems such as a leakage increase and seizure caused by friction.
As a measure against this thrust force, there are known a method for cancelling a thrust force by applying a discharge pressure to a gear end surface and a so-called two-set gear type pump in which gears are so coaxially arranged that twist directions thereof are opposite and respectively used as a pump.
Further, a gear pump which cancels a thrust force by using a double helical gear (JP1983-74885A) is also known.